Science —

Dinosaur-killing impact set the world on fire

Researchers argue forcefully that the ensuing heat set off global forest fires.

The mass extinction events that we understand well all seem to have relatively local causes: volcanic activity in Asia or Pangea, an impactor striking the Yucatan. But the events are so large that the environmental disruption goes global, killing off species across the planet and even in ecosystems, like the oceans, that might not be directly affected.

Massive volcanic eruptions can clearly create global destruction by dimming sunlight, causing sudden climate swings and acidifying the ocean. But it might be a bit harder to see how the impact of a large rock from space can reach into habitats halfway around the globe. Yet that's exactly what we think happened during the extinction that killed off all the non-avian dinosaurs (along with a host of other species). A paper in the Journal of Geophysical Research reviews the evidence for what its authors consider the most plausible model for global mayhem caused by the impact: its debris lit all the world's forests on fire at once.

The basic outlines of the scenario are pretty straightforward. The energy released by the impact sent lots of material high into and above the atmosphere, a lot of it near the escape velocity, which allowed it to spread around the planet. That is, after all, how we got the global layer of iridium that pointed to an impact being involved in the first place. But it's this material's return through the Earth's atmosphere that caused problems for vegetation. The heat of re-entry for all this material set off what's termed a "global infrared pulse."

It was quite an intense pulse, too; estimates are that there were as much as 10kg of debris that fell back through the Earth's atmosphere for every square meter of the Earth's surface. Preliminary calculations suggested that the resulting infrared pulse would be enough to set every piece of vegetation on the planet ablaze. "No non-aquatic vertebrate much larger than a squirrel survived," the authors note, and all the survivors could have plausibly burrowed underground to survive the conflagration.

It's a nice, simple explanation, but the real world is rarely quite that simple. The incoming radiation would have run into debris at lower altitudes that would have scattered it, diluting its impact. When this is fully modeled, it's enough to limit the impact so that only debris like fallen branches and pine needles would have been set alight. Still, that should be enough to set off a global conflagration.

The idea is plausible, but did it actually happen? Here's where the meat of the paper's arguments lie. Initial calculations suggest that the amount of soot in the debris layer was sufficient to indicate that "the entire terrestrial biosphere had burned." But at least some of that soot would have come from the carbon carried by the impactor itself, and some other fraction of it would have come from the ignition of organic material at or near the site of the impact. These factors can account for some of the soot in the debris layer, the authors argue, but require overly optimistic assumptions before they can account for all or even most of it.

Other arguments focus on the low amounts of charcoal at many sites, where the authors think the firestorms were intense enough to burn it off, and the presence of un-burned organic matter at others, which they ascribe to those sites initially being swampy environments.

To a certain extent, the paper zooms right past a rather important point: regardless of what, precisely, put the soot there, there was a whole lot of it in the atmosphere. So much, in fact, that modern climate models aren't even built with the capacity to model it. However, one attempt has been made to load a model atmosphere with 0.2 percent of the soot expected to have been put into the atmosphere. Even at that level, the global temperatures were predicted to have dropped to ice-age conditions quickly, and they stayed low for over a decade afterwards. Attempts to calculate the impact of the full atmospheric load suggest, at least initially, that no sunlight would have reached the surface of the Earth at all.

All told, a near-instaneous global conflagration, where nearly every bit of vegetation caught fire at once, doesn't seem very likely. But the authors build a strong case that large, perhaps global fires did eventually occur and were probably needed to create the amount of soot seen in the impact deposits.

The paper is meant to argue that global firestorms are the most likely result of the impact, caused by the heat from debris reentry. More generally, the paper makes it clear that the precise details probably aren't that important. There was clearly so much hot material in the atmosphere, and burning underneath it, that it would be surprising if there weren't some sort of extinction event.

It sounds plausible to my layman brain. The hardest point for the burning material to reach would be the exact opposite end of the impact. However, I've seen models (on the Discovery channel) that show that such a large impact would cause volcanic activity on the opposite end. So if you have 2 points burning and material ejected in between, it makes it a plausible argument in my non-expert opinion.

The cause of the extinction of the dinosaurs has fascinated me pretty much all of my life. It's interesting to read about more evidence that a meteor(s) impact would have been the primary cause.

As soon as I first heard about the extraterrestrial impact theory and the evidence for it discovered by Luis and Walter Alvarez, I immediately latched onto this idea since to me it matched the fossil record very well.- But strangely to my mind, major paleontologists seemed to cling to the concept of a gradual extinction in spite of the mounting evidence of a sudden catastrophe.

But the wheels of science keep grinding on even if it takes decades for acceptance by dinosaur experts.

Probably hyperbole to propose that all vegetation caught on fire. Somewhere had to be wet enough to not burn (or not burn much). The hydrologic cycle wouldn't have been immediately cancelled, either.

But we've known for a while now that essentially all large, terrestrial animals died out. So we know it was global and it was bad.

It is quite awe inspiring to imagine it being that bad, though. I always imagined the extinction event as something that happened over months/years after the impact, as food became scarce and temperatures fluctuated dramatically. The idea that the effect was much more immediate is rather shocking to picture. Basically, the result of that kind of fire activity would be that (except for lucky individuals or rare populations in protected areas) most large land animals would have died within minutes/hours of the impact.

The cause of the extinction of the dinosaurs has fascinated me pretty much all of my life. It's interesting to read about more evidence that a meteor(s) impact would have been the primary cause.

As soon as I first heard about the extraterrestrial impact theory and the evidence for it discovered by Luis and Walter Alvarez, I immediately latched onto this idea since to me it matched the fossil record very well.- But strangely to my mind, major paleontologists seemed to cling to the concept of a gradual extinction in spite of the mounting evidence of a sudden catastrophe.

But the wheels of science keep grinding on even if it takes decades for acceptance by dinosaur experts.

Paradigm shifts frequently occur only after the old guard has died off.

...killed off all the non-avian dinosaurs (along with a host of other species). ...all the survivors could have plausibly burrowed underground to survive the conflagration.

So the combination of those quotes implies the authors believe that the avian dinosaurs burrowed underground. That's surprising behavior for a flying animal.

Avian dinosaurs weren't really birds. Besides, flight motion (which might not have even emerged yet in the species) is similar to swimming motion (which would have been held over from prior species history), which is vaguely like burrowing motion (for which this event might have created extreme selective pressure).

But could these animals really have burrowed underground and stayed there until the forest fire burned out? Burrowing might have been a transient part of survival, but I think it more likely that these survivors were the animals that could have scurried out of the conflagration low enough to the ground to find breathable air.

The cause of the extinction of the dinosaurs has fascinated me pretty much all of my life. It's interesting to read about more evidence that a meteor(s) impact would have been the primary cause.

As soon as I first heard about the extraterrestrial impact theory and the evidence for it discovered by Luis and Walter Alvarez, I immediately latched onto this idea since to me it matched the fossil record very well.- But strangely to my mind, major paleontologists seemed to cling to the concept of a gradual extinction in spite of the mounting evidence of a sudden catastrophe.

But the wheels of science keep grinding on even if it takes decades for acceptance by dinosaur experts.

Edit due to mistakes caused by voice recognition

It was an extraordinary claim, an thus needed extraordinary evidence built up over time.

DanNeely wrote:

Paradigm shifts frequently occur only after the old guard has died off.

Consensus shifts surprisingly fast in science when evidence is clear and compelling.

Being from the Pacific Northwest, were I have personally witnessed over 1 month of solid rain - nonstop! I must come to the conclusion that the theory that the whole earth was set ablaze is faulty at best. Forest fires in the winter months of the rainy side of the Pacific Northwest are so rare (or extremely small and short lived) that it is easy to see that even if large portions of the earth were set ablaze by the impactor, there would still be large areas of the planet that would not be set ablaze.

This would not interfere with the theory of the killing off of a large majority of the terrestrial fauna as the amount of soot and debris in the atmosphere would be enough to seriously harm the whole earth ecosystem.

Maybe the authors of the paper were from a dry portion of the planet and don't understand just how wet large portions of the "dry land" of this planet really are?

Unless of course the atmospheric temperatures were kept relatively high to prevent the condensation of rainfall, but this seems countered by the observation that the soot caused an extreme drop in temperatures.

Being from the Pacific Northwest, were I have personally witnessed over 1 month of solid rain - nonstop! I must come to the conclusion that the theory that the whole earth was set ablaze is faulty at best. Forest fires in the winter months of the rainy side of the Pacific Northwest are so rare (or extremely small and short lived) that it is easy to see that even if large portions of the earth were set ablaze by the impactor, there would still be large areas of the planet that would not be set ablaze.

This would not interfere with the theory of the killing off of a large majority of the terrestrial fauna as the amount of soot and debris in the atmosphere would be enough to seriously harm the whole earth ecosystem.

Maybe the authors of the paper were from a dry portion of the planet and don't understand just how wet large portions of the "dry land" of this planet really are?

Well, I live in the Midwest and had close calls with grass fires 2 of the last 3 years. Had to evacuate my home during one, and the last thing I did was throw a sprinkler on the roof, one on either side of the house, turn all on, and beat feet. The fire line was stopped about 300 yards from the house, so I got lucky. Found out other folks did something similar - trying to wet down their homes against the fires. Most did not succeed as the fires, whipped by strong winds, just burned them anyway.

I think the article indicated burn outs in drier areas but the presence of carbon at what were 'wetter' areas like swamplands. Huge fires with lots of fuel and no containment - yeah, I'd think a global forest fire scenario is plausible.

Being from the Pacific Northwest, were I have personally witnessed over 1 month of solid rain - nonstop! I must come to the conclusion that the theory that the whole earth was set ablaze is faulty at best. Forest fires in the winter months of the rainy side of the Pacific Northwest are so rare (or extremely small and short lived) that it is easy to see that even if large portions of the earth were set ablaze by the impactor, there would still be large areas of the planet that would not be set ablaze.

This would not interfere with the theory of the killing off of a large majority of the terrestrial fauna as the amount of soot and debris in the atmosphere would be enough to seriously harm the whole earth ecosystem.

Maybe the authors of the paper were from a dry portion of the planet and don't understand just how wet large portions of the "dry land" of this planet really are?

Unless of course the atmospheric temperatures were kept relatively high to prevent the condensation of rainfall, but this seems countered by the observation that the soot caused an extreme drop in temperatures.

You will notice one thing in common for all of those species. They nest in burrows. And those are just the ones I could find in a couple minutes of googling.

Furthermore, all of the evidence seems to point to a small number of bird species surviving the event. So, yes, most birds do not burrow. But the hypothesis doesn't require all birds to be burrowers at the time. Doesn't even need most birds to burrow. It only needs "enough" burrowing species.

...killed off all the non-avian dinosaurs (along with a host of other species). ...all the survivors could have plausibly burrowed underground to survive the conflagration.

So the combination of those quotes implies the authors believe that the avian dinosaurs burrowed underground. That's surprising behavior for a flying animal.

Are people really that unaware of birds that dig burrows? There's a species named the burrowing owl, and it's hardly the only one that does that.

Dude, I know significantly more about biology than the average person, recently specialized in birds, and still didn't think of burrowing owls, cliff swallows and other birds that live in burrows. No need to be a dick.

...killed off all the non-avian dinosaurs (along with a host of other species). ...all the survivors could have plausibly burrowed underground to survive the conflagration.

So the combination of those quotes implies the authors believe that the avian dinosaurs burrowed underground. That's surprising behavior for a flying animal.

Are people really that unaware of birds that dig burrows? There's a species named the burrowing owl, and it's hardly the only one that does that.

I am aware that some birds are burrowers, but they're a pretty small subset. The inference of this article is that pretty much all of the avian dinos that survived must have been burrowers which seems like quite a stretch. I haven't shelled out the bucks to read the whole paper, but it seems they may also be glossing over the fact that essentially every food source for terrestrial life would have been reduced to ash.The idea of widespread fires is hardly novel, but a global conflagration introduces more theoretical problems than it solves.

if it was that bad, it would seem like oxygen levels would quickly get too low to support forest fires. they might smoulder over a long time rather than burn for a short time. secondarily, species that could adapt to low oxygen levels would have an advantage.

That would be consistent with the conclusions of Beerling and Co. (2002, link: http://www.pnas.org/content/99/12/7836.full.pdf) who found that CO2 concentrations rose by 2,300 ppm within 10,000 years (in context, Global Warming is an increase of ~100ppm over the past century). The oceans would likely be a key uptake for CO2, which (imo) would explain the oceanic extinctions of critters such as ammonites at the time.

...killed off all the non-avian dinosaurs (along with a host of other species). ...all the survivors could have plausibly burrowed underground to survive the conflagration.

So the combination of those quotes implies the authors believe that the avian dinosaurs burrowed underground. That's surprising behavior for a flying animal.

Are people really that unaware of birds that dig burrows? There's a species named the burrowing owl, and it's hardly the only one that does that.

I am aware that some birds are burrowers, but they're a pretty small subset.

Well, only a pretty small subset of bird species seem to have survived the event.

But it is likely that some avian species had other behaviors that provided similar levels of protection. Cliff-dwelling species would not have been particularly susceptible to forest fires. But even then, you'd imagine that species that burrow into sedimentary cliff faces would have an advantage over species that live on the crags of igneous cliff faces. Burrows would protect against the direct effects of that initial infrared pulse. If it's hot enough to set forest ablaze, it might have been hot enough to set feathers/fur ablaze, as well.

What about the atmosphere becoming too hot for most species to survive? I seem to remember reading that there was a theory suggesting that most of the life essentially roasted in the superheated atmosphere which was followed by fallout-like cooling due to ashes in the air that killed most of the life around the globe.

if it was that bad, it would seem like oxygen levels would quickly get too low to support forest fires. they might smoulder over a long time rather than burn for a short time. secondarily, species that could adapt to low oxygen levels would have an advantage.

Too much oxygen and not enough vegetation for that to happen. But the ratios of oxygen and CO2 in the atmosphere would have changed dramatically, just not dramatically enough to actually cause the fires to smoulder.

It was my understanding non-avian dinosaurs were already on a slow path to extinction if they didn't evolve before the impact. The reason, they didn't adopt their diets to include flowering plants (angiosperms) like grass. The impact/volcano events just sped up the death of thousands of species of the plants they ate. They starved to extinction.

Is that not the case? (kind of like how you don't die from being set on fire from the heat itself, you die from asphyxiation)

For avian survival, what about sea birds? It's hard to set the ocean on fire, and even if you do drop 10 kg of hot debris into every square meter you're only going to warm it up slightly. The specific heat of the ocean is enormous.

Of course that raises the question about "if the ocean is not so bad, what killed off all the plesiosaurs and ocean-fishing pterosaurs?" Some combination of the subsequent ice age and ecosystem collapse, I would guess? Plus of course locally you do boil the entire future-Carribbean...

But back to the main point: my god, a nearly global firestorm. Terrifying to think about. I feel bad for the folks in Russia that got their windows smashed in by that meteor but... there's just no words for how epically astoundingly terrible that is to even contemplate. .

The problem with these types of analyses is that everyone seems to look for the "all or nothing" angle in favor or against. Why? Would it have been necessary for "every piece of vegetation in every part of the world to catch fire" in order for all of the land-dwelling species over a certain size to quickly die out? No. The animals themselves (and the vegetation), was probably distirbuted in a somewhat random way.

If many of those of those random areas (instead of "everything everywhere") where animal populations were high, were torched, that would be sufficient to kill most of them off and the ensuring lack of sun / food, and/or ice age would kill the rest. IE: Some species probaby died in the immediate impact and aftermath, some from the fires on other side of the world, some from starvation, etc.

For avian survival, what about sea birds? It's hard to set the ocean on fire, and even if you do drop 10 kg of hot debris into every square meter you're only going to warm it up slightly. The specific heat of the ocean is enormous.

Of course that raises the question about "if the ocean is not so bad, what killed off all the plesiosaurs and ocean-fishing pterosaurs?" Some combination of the subsequent ice age and ecosystem collapse, I would guess? Plus of course locally you do boil the entire future-Carribbean...

Ocean acidification that comes with rising CO2-levels does very bad things to marine ecosystems. Add lack of sunlight and a collapse of all the tiny creepers that sit at the very start of the food-chain and things aren't looking good for anything in the water.

The interesting thing is that right now we're living in a mass extinction event and can study all of its details...